cout statement change program semantics

Question

I have the following piece of code, which is part of a class, whose details hopefully are not important for answering that question. I don't want an exact solution to my problem, but only an explanation, how the behavior, I'm going to describe, can occur.

    Character operator+(Character c) {
        std::vector<float> resDensity(density);
        float l;
        float dh;
        for (int i = 0; i < domains.size(); i++) {
            for (int j = 0; j < c.domains.size(); j++) {
                l = (domains[i].intersection(c.domains[j])).length();
                //std::cout << l;
                dh = c.density[j] * l / domains[i].length();
                resDensity[i] += dh;
            }
        }
        std::vector<Domain> h(domains);
        return Character(h, resDensity);
    }

You probably noticed the commented cout statement. Due to a numeric bug in my program, I followed the falsy value until I detected the variable l . So I printed it to the console and found out, that the value is exactly the value I need, and also the program works just fine and the bug simply vanished. Uncommenting it, again, leads to the undesired misbehavior.

The Character class contains the fields domains and density , both vector s...

The length and intersection method:

    float length() {
        float res;
        for (int i = 0; i < is.size(); i++) {
            res += is[i].b - is[i].a;
        }
        return res;
    }

    Domain intersection(Domain d) {
        std::vector<Interval> hi;
        for (auto i = is.begin(); i != is.end(); i++) {
            for (auto j = d.is.begin(); j != d.is.end(); j++) {
                hi.push_back(i->intersection(*j));
            }
        }
        Domain hhh(hi);
        return hhh;
    }

I compiled the code with g++ --std=c++11 ...

g++ --version
g++ (GCC) 4.8.2 20131219 (prerelease)

Answer 1

In your length() function the res variable is uninitialized:

    float res;
    for (int i = 0; i < is.size(); i++) {
        res += is[i].b - is[i].a;   // <-- uses an uninitialized res
    }

Answer 2

How is length() calculated on Domain ? hhh is a local variable that you are returning, so a copy will be made. If length() depends on something that is not copied then you will see strange behaviour. What if you just change intersection() to return a float or create a different method called intersectionLength() which returns a float - do you still get the heisenbug?